Yin and Yang yīn yáng · 阴阳

Yin and Yang are not good versus evil, and not two separate things. They are two complementary aspects of one phenomenon. Yin is the cooler, quieter, interior, resting, material side; Yang is the warmer, more active, exterior, moving, functional side.

Health in this model is their relative balance — and, more importantly, their ability to keep turning into one another. Every other TCM concept (cold and heat, excess and deficiency, interior and exterior) is built on this pair.